The present invention relates to a video display apparatus.
There has been known a video display apparatus that comprises an ABL circuit and/or a contrast adjustment circuit. There has particularly been known an ABL which controls the display apparatus so as to prevent an average display brightness on a display screen from becoming excessively high for the purpose of suppressing concentration of a beam and power consumption or for other purposes.
[Patent Document 1]
Japanese Patent Application Laid-Open No. 2000-221941
In Patent Document 1, a construction for performing the ABL is disclosed. In this construction, an automatic brightness control circuit is used to adjust a brightness level for pixel data for each pixel which are sequentially supplied from an A/D converter in order for an average brightness of an image to be displayed on a display screen of a PDP to come within a predetermined brightness range. The adjustment of brightness level here is carried out before the inverse gamma correction is performed under setting of a ratio of the number of light emissions in the respective sub-fields to an non-linearity.
In general, a video signal is subjected to a non-linear conversion such as a conversion of the 0.45th power of the signal as shown in FIG. 11, called as the gamma conversion, which is adapted to an input vs. light emission characteristic of a CRT display, assuming that the video signal is displayed by a CRT display apparatus, and the subjected video signal is transmitted or recorded. When such a video signal is displayed for displays having a linear input vs. light emission characteristic in a SED, FED, PDP or the like, an input signal is subjected to the inverse gamma conversion such as a conversion of the 2.2th power of the signal as shown in FIG. 12. If displaying based on other input vs. light emission characteristics is performed in an LCD device or the like, a conversion suitable to the display device is carried out.
FIG. 13 shows a block diagram of a video signal processing apparatus amounted on a typical video display apparatus using a display device such as SED, FED, PDP, LCD having a characteristic of light emission responsive to an input signal which is different from that of a CRT. Although an actual video signal processing apparatus is comprised of many other processing circuits, this figure shows only blocks relating to the present invention.
The video signal processing apparatus shown in FIG. 13 has an A/D converter 1001, a signal processing section 1002, an average brightness detecting section 1003 and a gain calculating section 1004. The A/D converter 1001 receives an analog input video signal s101 and outputs a digital video signal s102. The signal processing section 1002 receives the digital video signal s102, and subjects the received signal to a signal processing such as an inverse gamma conversion, brightness and chromaticity adjustments, an edge enhancement processing to output a display signal s103. The average brightness detecting section 1003 receives the digital video signal s102, and detects an average brightness for each frame to output an average brightness signal s104. The gain calculating section 1004 receives the average brightness signal s104 and outputs a brightness control signal s105.
The A/D converter 1001, signal processing section 1002, average brightness detecting section 1003 and gain calculating section 1004 respectively operates on the basis of various kinds of timing signals generated from a synchronization signal of the input video signal s101 by a timing control section, not shown.
However, in the configuration shown in FIG. 13, an average brightness is obtained by totaling values for the digital video signal s102, but nevertheless an image actually displayed in the display apparatus is originated from the display signal s103 obtained by subjecting the digital video signal s102 to the different signal processings including an inverse gamma conversion. Especially the inverse gamma conversion is intended to carry out a non-linear conversion such as the 2.2th power of the signal as shown in FIG. 12, and therefore the average brightness of the signal is significantly decreased, so that a one-to-one correspondence is collapsed between an average brightness of the digital video signals s102 and an average brightness of the display signal s103 that has been converted in the inverse gamma conversion. For these reasons, the average brightness signal s104 detected in the average brightness detecting section 1003 has some error with respect to an average brightness actually provided for display in the display apparatus, whereby accurate average brightness information could not been obtained.